AN INTRODUCTION TO SPINTRONICS - PowerPoint Presentation

Slide1

AN INTRODUCTION TO SPINTRONICS

BY: SAMIR KUMAR10M601M.TECH 1ST YEARCenter for Materials Science and Engineering

NATIONAL INSTITUTE OF TECHNOLOGYHAMIRPURCentre for Materials Science and Engineering

रास्ट्रीय प्रद्योगिकी संस्थान हमीरपुरSlide2

OutlineIntroduction

What do we mean by spin of an electronWhy SpintronicsSpintronic EffectsPhases in Spintronics Materials of SpintronicsConclusionsAcknowledgmentsSlide3

Electron has :

MassChargeSpin INTRODUCTIONSlide4

What is spin?

One can picture an electron as a charged sphere rotating about an axis. The rotating charged sphere will produce magnetic moment in that can be either up or down depending upon whether the rotation is anticlockwise or clockwiseSlide5

Electron Spin is a Quantum phenomenon

A spinning sphere of charge can produce a magnetic moment.Considering Electrons size to be of the order of 10-12 m at that size a high spin rate of some 1032 radian/s would be required to match the observed angular momentum that is velocity of the order of 1020 m/s.Slide6

The component

S

z along z axis:Electron SpinSlide7

Conventional electronic devices ignore the spin property. Random spins have no effect on current flow. SPINTRONICS = SPIN + ELECTRONICSWhat is Spintronics?

Spintronics=spin based electronicsSpintronic devices create spin-polarized currents and use the spin to control current flow.Slide8

Moore’s Law

Moore’s Law states that the number of transistors on a silicon chip will roughly double every eighteen monthsWhy Spintronics?Slide9

Can Moore’s law keep going?

Power dissipation=greatest obstacle for Moore’s law! Modern processor chips consume ~100W of power of which about 20% is wasted in leakage through the transistor gates. The traditional means of coping with increased power per generation has been to scale down the operating voltage of the chip but voltages are reaching limits due to thermal fluctuation effects. Slide10

Advantages of Spintronics Devices

Non-volatile memory Performance improves with smaller devices Low power consumption Spintronics does not require unique and specialised semiconductorsDissipation less transmission

Switching time is very lessCompared to normal RAM chips, spintronic RAM chips will:– increase storage densities by a factor of three– have faster switching and rewritability rates smallerPromises a greater integration between the logic and storage devicesSlide11

Spintronics Effects

GMR (Giant Magneto-Resistance)FM-Metal-FMMTJ (Magnetic Tunnel Junction)FM-Insulator-FMSlide12

Giant Magneto-Resistance (GMR)

The 2007 Nobel prize for physics was award jointly to Fert and Grunberg for giant magnetoresistance (GMR) discovered independently in 1988.

This discovery led to development of the “spin valve” and later the tunnel magnetoresistance effect (TMR) which found application in advanced computer hard drives, and more recently magneto-resistive random access memory (MRAM) (which is non-volatile). Slide13

Giant Magneto-Resistance (GMR)

Discovered in 1988 FranceA

multilayer GMR consists of two or more ferromagnetic layers separated by a very thin (about 1 nm) non-ferromagnetic spacer (e.g. Fe/Cr/Fe)When the magnetization of the two outside layers is aligned, resistance is low

Conversely when magnetization vectors are antiparallel, high R

Condition for GMR: layer thickness ~ nmSlide14

Parallel Current GMR

Current runs parallel between the ferromagnetic layersMost commonly used in magnetic read headsHas shown 200% resistance difference between zero point and antiparallel statesSlide15

Perpendicular Current GMR

Easier to understand theoretically, think of one FM layer as spin polarizer and other as detectorHas shown 70% resistance difference between zero point and antiparallel statesBasis for Tunneling MagnetoResistanceSlide16

Concept of the Giant Magnetoresistance (GMR)

1) Iron layers with opposite magnetizations : spin up and spindown are stopped → no current (actually small current only)

2) If a magnetic field aligns the magnetizations: spins go throughSlide17

Applications of GMR

It is used in Hard Drives0.5 MB← 19751997 (before GMR) : 1 Gbit/in

2 , 2007 : GMR heads ~ 300 Gbit/in2

100 GB hard disc (Toshiba), →soon in portable digital audio-playersSlide18

Magnetic Tunnel Junction

A magnetic tunnel junction (MTJ) consists of two layers of magnetic metal, such as cobalt-iron, separated by an ultrathin layer of insulator.Tunnel Magnetoresistive effect combines the two spin channels in the ferromagnetic materials and the quantum tunnel effect

FerromagneticelectrodesSlide19

Magnetic Tunnel Junction

Device

Parallel alignment (P)

Antiparallel alignment (AP)

Ferromagnetic leads

L

&

R

Insulating spacer

S

Measured

: tunneling current

I

, conductance

G

Tunneling magneto-resistance (TMR)Slide20

Applications

The read heads of modern hard disk drives.Is also the basis of MRAM, a new type of non-volatile memory.Slide21

Magnetoresistive

Random Access MemoryMRAM uses magnetic storage elements instead of electric used in conventional RAMTunnel junctions are used to read the information stored in Magnetoresistive Random Access Memory, typically a ”0” for zero point magnetization state and “1” for antiparallel stateSlide22

MRAM combines the best characteristics of Flash, SRAM and DRAMSlide23

Phases in Spintronics

SPIN INJECTIONSPIN MANIPULATIONSPIN DETECTIONSlide24

Spin injection

It is the transport or creating a non-equilibrium spin population across interface

Using a ferromagnetic electrode

Effective fields caused by spin-orbit interaction.Tunnel barrier could be used to effectively inject spins into a semiconductorTunneling spin injection via Schottky barrierBy “hot” electronsSlide25

Spin Manipulation

To control electron spin to realize desired physical operation efficiently by means of external fieldsMechanism for spin transfer implies a spin filtering process.Spin filtering means that incoming electrons with spin components perpendicular to the magnetic moment in the ferromagnet are being filtered out.Spin-polarized current can transfer the angular momentum from carriers to a ferromagnet where it can change the direction of

magnetization This effect is equivalent to a spin transfer torque.Slide26

Spin Transfer Torque

The spin of the conduction electron is rotated by its interaction with the

magnetization.This implies the magnetization exerts a torque on the spin. By Conservation of angular momentum, the spin exerts an equal and

Opposite torque on the magnetization.

S

v

vSlide27

Spin Detection

To measure the physical consequences of spin coherent states in Spintronics devices.The injection of non-equilibrium spin either induces voltage or changes resistance corresponding to buildup of the non-equilibrium spin. This voltage can be measured in terms of change in resistance by potentiometric method.Slide28

Spin Detection Technique

An ultrasensitive silicon cantilever with a SmCo magnetic tip positioned 125nm above a silica specimen containing a low density of unpaired electron

spins. At points in the specimen where the condition for magnetic resonance is satisfied, the magnetic force exerted by the spin on the tip.Slide29

Materials of Spintronics

Currently used materials in conventional electronics are usually non-magnetic and only charges are controllable.Existing metal-based

devices do not amplify signals. Whereas semiconductor based spintronic devices could in

principle provide amplification and serve, in general, as multi-functional devices.All the available ferromagnetic semiconductor materials that can be used as spin injectors preserve their properties only far below room temperature, because their Curie temperatures (TC) are low.

ProblemsSlide30

GMR - Giant magnetoresistance -

HDD read headsMTJ - Magnetic Tunnel Junction - HDD read heads+MRAMMRAM - Magnetic RAM - nonvolitile memorySTT - Spin Transfer Torque - MRAM+oscillator

Spintronic Research and ApplicationsSlide31

Solution

Diluted Magnetic Semiconductor or (DMS).Add Fe or Mn toSi/GaAsHalf-Metallic Ferromagnets

Fe3O4 magnetite CrO2Heusler FM

Ni2MnGaCo2MnAlSlide32

Diluted Magnetic Semiconductor or (DMS)

One way to achieve FS is to dope some magnetic impurity in a semiconductor matrix. (Diluted Magnetic Semiconductor )

Semiconductor host atom

Magnetic impuritySlide33

Theoretical predictions

by

Dietl, Ohno et al.

Various DMS displays room temperature ferromagnetism!

Curie Temperature

— The temperature above which a

ferromagnetic

material loses its permanent magnetism.

Science

287

, 1019 (2000)

& PRB

63

, 195205 (2001

) Slide34

DMS materials I: (Ga,Mn)As

First DMS material, discovered in 1996 by Ohno et al.Curie temperature K at optimal doping

Max TC ~ 110Kx ~ .05

[Ohno et al., APL 69, 363 (1996)]Slide35

DMS materials II: (Ga,Mn)N

First room temperature DMS discovered in 2001High curie temperatureExperiment: up to Tc =800 K

Theory: up to Tc =940 K

Highest Tc

in

Dietl’s

predictionSlide36

DMS materials III:

Transition metal doped oxideRoom temperature ferromagnetism discovered in Mn doped ZnO in 2001Material:Mn doped ZnO

Co doped TiOReported Tc up to 400K

Hysteresis curve at Room temperature for

Mn

doped

ZnO

(

Sn

) Slide37

Half-Metallic Ferromagnets

Half metals are ferromagnets with only one type of conduction electron, either spin up, ↑, or spin down, ↓The valence band related to one type of these electrons is fully filled and the other is partially filled. So only one type of electrons (either spin up or spin down) can pass through it.Slide38

Half-Metallic Ferromagnets

E.g.:Chromium(IV) oxideFe3

O4 magnetite Heusler alloysSlide39

Future Outlook

High capacity hard drivesMagnetic RAM chipsSpin FET using quantum tunnelingQuantum computers Slide40

Limitations

Problems that all the engineers and scientists may have to overcome are:To devise economic ways to combine ferromagnetic metals and semiconductors in integrated circuits.To find an efficient way to inject spin-polarized currents, or spin currents, into a semiconductor. To create long relaxation time for effective spin

manipulation.What happens to spin currents at boundaries between different semiconductors?How long can a spin current retain its polarization in a semiconductor?Slide41

THANK YOU

for your kind attention

☺